1. Field of the Invention
The present invention relates to an image heating apparatus such as a fixing device mounted in an image forming apparatus such as a copier or printer, and an image forming apparatus having the image heating apparatus.
2. Related Background Art
With recent trends in the design of image forming apparatuses such as electrophotographic printers toward high-speed, multifunctional, full-color features, various types of printers with these features have already hit the market. From the high-speed printer""s point of view, research and development of image forming apparatuses adopting an in-line system have made progress. The in-line system is such that a plurality of electrophotographic units different in color with one another are so arranged in series that they will be driven at the same time, which makes it possible to form color images faster. Such in-line type image forming apparatuses have great potential for business use.
Because of less process components, which make it easy to provide a compact, cheap apparatus, many conveying belt type in-line printers have been developed. The conveying belt type in-line system is such that a plurality of electrophotographic units, each of which incorporates plural electrophotographic processes such as electrification, exposure, development and cleaning therein, are so disposed that they sequentially overlap and transfer toner images onto a transferring material (recording paper) while sucking the transferring material on a conveying belt that also serves as a transfer belt.
FIG. 5 illustrates a structure of an in-line type image forming apparatus. In the image forming apparatus, process stations 20M, 20C, 20Y and 20K for yellow (Y), magenta (M), cyan (C) and black (K) are placed horizontally in a line along the circumferential surface of a transferring material conveying belt (ETB) 1 wound around a driving roller 2 and a tension roller 3.
In each process station 20 (20M-20K), an electrifier 7 uniformly electrifies the surface of a photosensitive drum 6, and an exposing optical system 8 forms a latent image on the surface. The latent image is developed by a developing device 9 and visualized as a toner image. The toner image in each color formed on the photosensitive drum 6 is transferred by a transferring roller 4 to the surface of a transferring material on a conveying belt 1 in a transferring portion opposite to the photosensitive drum 6, thus superimposing four-color toner images on the transferring material to form a color image. Residual toner remaining on the photosensitive drum 6 after the transfer of each toner image is scraped by a cleaner 10, cleaning up the surface of the photosensitive drum 6.
The transferring material is fed from a sheet feeding cassette 14 to the conveying belt 1 by means of pickup rollers 15. Then, when passing through a nip portion formed between an absorbing roller 5 and the tension roller 3, the transferring material is applied with a bias voltage and electrostatically attracted to or absorbed on the conveying belt 1. The transferring material absorbed on the conveying belt 1 is conveyed in the horizontal direction of arrow by means of the conveying belt 1.
The transferring material to which four-color toner images have been transferred is separated from the conveying belt 1 in a downstream rear-end portion of the conveying belt 1 along a curvature, and conveyed to a fixing apparatus 11 by which the toner images are fixed. The fixing apparatus 11 includes a heating roller 121 with a halogen heater 121a, a fixing part 12 with a pressure roller 122 made in contact with the heating roller 121, and a sheet discharging part 13 consisting of a pair of sheet discharging rollers 131 and 132.
While the transferring material is being nipped and conveyed by a fixing nip portion between the heating roller 121 and the pressure roller 122, the four-color toner images are fixed by the application of heat and pressure. The transferring material on which the toner images have been fixed by the fixing part 12 is delivered to the outside of the fixing apparatus 11 by means of the sheet discharging part 13, and finally placed or stacked on a sheet discharging tray 16.
In addition to the above-mentioned horizontally conveying type in-line printer, a vertically conveying type in-line printer has also been developed for the purpose of reducing the installation area. The vertically conveying type in-line printer assumes such a form that the horizontally conveying type in-line printer of FIG. 5 is rotated 90 degrees. In the horizontally conveying type in-line printer, the transferring material absorbed on the transferring belt is conveyed upward against gravity while transferring toner images in respective colors from respective process stations to the transferring material, thus forming a full-color image. The full-color image is then heat-fixed by the fixing apparatus placed at the upper side of the apparatus.
One of salient features required for color printers is the transparency of an image to be printed out on an OHP (Over Head Projector) sheet.
The OHP sheet is a synthetic resin film, for example, PET film, of about 100 xcexcm in thickness. Since the OHP sheet has a larger heat capacity than normal paper, it needs to be fixed by a larger amount of heat than usual. In order to achieve excellent transparency, it is necessary to fuse toner images on the OHP sheet securely enough to make the surface of the toner images smooth. Therefore, upon fixing the toner images transferred to the surface of the OHP sheet, the fixing temperature is increased, or the conveying speed during a fixing period is slowed down so that the amount of heat to be applied per unit time will increase.
In the horizontally conveying type in-line printer, where the electrophotographic units are placed in the horizontal line, the fixing apparatus and the conveying belt are arranged side by side, positioning the heating roller on the pressure roller in the fixing apparatus. Therefore, as shown in FIG. 6, an OHP sheet S softened by heat applied when passing through the fixing nip portion turns downward in a delivery direction by its own weight, that is, toward the pressure roller 122 side, and discharged to the outside of the apparatus by means of pair of sheet discharging rollers 131, 132 through a bent path as indicated by the solid line Ts.
On the other hand, in the vertically conveying type in-line printer, where the electrophotographic units are placed in the vertical line, the fixing apparatus 11 is placed above the conveying belt, and as shown in FIG. 7, the sheet discharging part 13 is positioned right above the fixing part 12 in the fixing apparatus 11. In this case, since the OHP sheet S is conveyed substantially in the vertical direction in the fixing nip portion, such an effect of the horizontally conveying type that it lets the OHP sheet turn its delivery direction down toward the pressure roller 122 side by its own weight. As a result, as shown in FIG. 7, the delivery direction might unsteadily form, other than a path T0, a path T1 bent on the heating roller 121 side or a path T2 bent on the pressure roller 122 side.
If the OHP sheet passes through the path T1 on the heating roller 121 side, one side of the OHP sheet on which toner images are borne is brought into contact with the heating roller 121 for a long time. In this case, it was found that the smoothness of the surface of the toner images would be spoilt, resulting in occurrence of such a phenomenon as to make the transparency of the resulting image fixed on the OHP sheet worse (hot offset).
The hot offset can be prevented by lowering the fixing temperature. In this case, however, if the OHP sheet S forms the path T2 bent on the pressure roller 122 side, toner is not melted enough, thereby making the transparency worse.
To eliminate the formation of the above-mentioned paths, pressure between the sheet discharging rollers 131 and 132 can be so increased that the OHP sheet S after passing through the fixing nip portion is kept pulled to take the path T0. In this case, however, it was also found that the increase in the pressure between the sheet discharging rollers spoilt the smoothness of the surface of the toner images on the OHP sheet, and hence made the transparency worse.
As discussed above, although the vertically conveying type in-line printer has the advantage of reducing the installation area of the apparatus, it was found that it tended to lower the transparency of the toner images on the OHP sheet, compared to the horizontally conveying type, unless the sheet discharging angle was aggressively determined.
The present invention has been made in consideration of the above-mentioned conventional problems, and it is an object thereof to provide an image heating apparatus and an image forming apparatus provided with the image heating apparatus capable of preventing occurrence of a fixing failure.
It is another object of the present invention to provide an image heating apparatus and an image forming apparatus provided with the image heating apparatus capable of forming on a transparent recording material an image with excellent transparency.
It is still another object of the present invention to provide an image heating apparatus comprising:
a heating member;
a backup member that cooperates with the heating member to form a nip for nipping and conveying a recording material; and
a conveying member provided downstream of the nip in a traveling direction of the recording material,
wherein the conveying member is provided closer to the backup member than the heating member.
It is yet another object of the present invention to provide an image forming apparatus comprising:
fixing means having a nip for nipping and conveying a recording material, the fixing means including a heating member and a backup member that cooperates with the heating member to form the nip for nipping and conveying the recording material; and
a conveying member provided downstream of the nip in a traveling direction of the recording material,
wherein the conveying member is provided closer to the backup member than the heating member.
Further and other objects of the present invention will become apparent from reading the following detailed description in connection with the accompanying drawings.